This post exists mainly to update the account I gave in Rationality and Morality.
Korsgaard in “The Sources of Normativity” proposes a motivation to be rational that is not based on either an instinctive urge or some eternal force. Instead the argument for rationality is motivated by the structure of our consciousness. She argues that because we are self-conscious beings we maintain a mental representation of our selves. This mental representation is not simply a collection of facts about our lives, but rather (at least in part) a set of rules that describes why we act in general. When our actions conform to our mental representations of ourselves we are happy, and we become upset when we act in ways that conflict with this image. When we take an action then we shouldn’t consider our desires alone; we should also take into account the reason for action and whether that reason fits with our self image. Korsgaard would argue then that someone who acted simply on their desires would only be satisfied if their self description contained the rule “I act only to satisfy my desires”. Since most people do not describe themselves in this way, egoistic accounts of action can be rejected as not actually reflecting how the majority of the population is motivated to act.
Unfortunately this reasoning cannot get us to the strong rationality required by Nagel and Kant. It does show that a person should act according to some scheme and that their rules should be consistent with each other; otherwise they may find themselves in a situation where they are unable to live up to their own standards. However the stronger version of rationality would require us to act only in ways that can be justified. In contrast the rules that a person decides to live by do not have to be justified, they simply have to exist. It is possible under Korsgaard’s account that someone may live egoistically, and thus there is no necessary motivation to be strongly rational. As long as the rules that the altruist and egoist live by are consistent they can be equally satisfied with themselves.
Note: Korsgaard did attempt to show that strong rationality followed from her account, by arguing that living by our self imposed rules implied we had reason to value our own humanity. From valuing our own humanity she argued we are then motivated to value humanity in general. Unfortunately this move cannot be justified; it requires that we are compelled to act in a strongly rational manner already (i.e. since I value my own humanity I have no reason not to value the humanity of others). That would be a circular argument, and without an assumption of strong rationality there is no motivation driving a person not to act as if their own humanity was special, or to simply doubt that others were equally as human.